Linear transmitters are known and are widely used in today's information age. A Cartesian feedback transmitter constitutes one such known transmitter. A Cartesian feedback loop is used to compensate for nonlinearities in a power amplifier of a Cartesian feedback transmitter. Loop phase of the Cartesian feedback loop must be properly adjusted; if not, the Cartesian feedback transmitter linearity may suffer. At worst, the feedback of the Cartesian feedback loop may function in a positive mode, causing undesirable oscillations.
One approach for adjusting phase within the Cartesian feedback loop employs a frequency domain technique. The frequency domain technique uses complex frequency mixers and quadrature generators to generate a phase shifted local oscillator (LO) signal, which introduces a controlled phase shift into the Cartesian feedback loop. The frequency domain technique uses mixers to mix sine and cosine components with a LO signal to shift the phase of the LO signal.
While the frequency domain technique has been effective, there are some limitations which still persist. First, due to harmonics created by phase shifting circuitry, filtering is required in the LO signal path to enable the transmitter to operate in a stable mode over multiple frequency bands. The wider the range of frequencies over which the transmitter is required to operate, the more filter components are required, which consumes space and current drain and adds complexity to the transmitter circuit.
Therefore, as we move towards multiple mode and multi-band technology, such as the next generation software definable radios; there is a need for an improved method and apparatus for generating phase shifted local oscillator signals for a feedback loop in a transmitter.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.